Epoxy resins have been used for various purposes such as paints, electric or electronic insulating materials and adhesives because hardened products of them are excellent in mechanical properties, electrical properties, thermal properties, chemical resistance, adhesion properties, and the like.
As an epoxy resin composition used for such purposes, so-called two-component epoxy resin compositions (which may also be called “two-part epoxy resin compositions”) comprising an epoxy resin and a hardener and cured by mixing these two components upon use are typical. Two-part epoxy resin compositions can be cured well at room temperature, but need separate storage of the epoxy resin and the hardener. In addition, upon use, they must be weighed and then mixed with each other. In addition, a mixture of the epoxy resin and the hardener is usable for only a limited time after mixing so that it is impossible to prepare a large amount of the mixture in advance. In short, there is room for improvement in the conventional two-part epoxy resin composition from the viewpoint of easy storage, easy handling, and mixing frequency (production efficiency).
Some one-component epoxy resin compositions (which may be referred to as “one-part epoxy resin compositions) have been proposed. Examples of such one-part epoxy resin compositions may include those obtained by incorporating, in an epoxy resin, a latent hardener such as dicyandiamide, BF3-amine complex, an amine salt, or a modified imidazole compound.
However, these one-part epoxy resin compositions are, when excellent in storage stability, inferior in hardening property (treatment at high temperatures or treatment for long hours is necessary for hardening). On the other hand, they are, when excellent in hardening property, inferior in storage stability (storage at a temperature as low as −20° C. is demanded). For example, a one-part epoxy resin composition containing dicyandiamide can be stored stably for 6 months or greater when stored at normal temperature. Such a one-part epoxy resin composition, however, sometimes needs a hardening temperature as high as 170° C. or greater. Addition of a curing accelerator to such a one-part epoxy resin composition can decrease the hardening temperature to approximately 130° C. It is apt to deteriorate the storage stability at room temperature so that storage at low temperature is required. In short, there is an eager demand for the development of a one-part epoxy resin composition capable of satisfying high hardening property and excellent storage stability.
In order to satisfy such a demand, a so-called microcapsule-based hardener obtained by covering a core containing an amine hardener with a specific shell has been proposed (refer to, for example, Patent Document 1 and Patent Document 2). The microcapsule-based hardeners disclosed therein can satisfy both good hardening property and storage stability.    [Patent Document 1] Japanese Patent Laid-Open No. Hei 1-70523    [Patent Document 2] Japanese Patent Laid-Open No. 2005-344046